This application relates to a process for treating liquid and solid wastes from the tanning and trimming process of tanneries which convert hides to leather.
In tanneries, prepared hides are treated with one or more of a variety of chemicals to strengthen the leather by a curing or cross-linking of the protein content thereof and to extend the useful life of the leather by preventing putrification. The term "tanning" has become a term of art to describe this chemical treatment due to the fact that tannins were originally employed to cure the hides.
For a considerable period of time, however, chromium compounds have been utilized to achieve tanning of hides. Chromium compounds have the capacity for forming strong coordination bonds and thus stable cross-links between adjacent protein (collagen) molecules.
The cost of the chromium compounds employed in conventional tanning operations is a significant factor in the overall cost of the tannery operation. Significant amounts of chromium values are lost in the liquid waste resulting from the tanning operation. Additionally, large quantities are lost upon disposal of leather waste such as scrap from trimming operations, etc.
Early efforts in the tanning industry to deal with liquid and solid tannery wastes did not focus on recovery of chromium values, but, rather, on disposal of the waste material as landfill, discharge into surface waters, etc. Inasmuch as chromium compounds are toxic, environmental considerations have demanded alternative routes for disposal of such wastes. In addition, efficiency and cost considerations have demanded alternative routes which would enable the recovery of chromium values from the waste material.
The Environmental Protection Agency has been particularly sensitive to problems and control of waste materials in landfills. Tannery leather scrap and treatment sludges are considered potentially hazardous wastes. These wastes, disposed of in landfills, can cause leachate problems for extended periods of time since their rate of degradation is very slow. Where a chromium compound is used for tanning, even more difficult waste disposal problems result due to the toxic nature of the chromium salts. Generally, the chromium compounds are discharged in untreated liquid effluents. It is the disposal of these wastes that gives rise to a major environmental problem for the industry.
The waste disposal problems and the increase in energy and material costs have combined to depress the tanning industry in the United States.
It has been suggested to recover chrome values from liquid tannery wastes through chrome precipitation by raising the pH thereof (Pierce, "Recycling of Chrome Tanning Liquors", J. Am. Leather Chem., Vol. 71 (4), page 161 (1976), or by pickling with cuprous material in an aqueous sulfuric acid medium (Robinson et al, U.S. Pat. No. 3,822,993).
U.S. Pat. No. 3,950,131 to Young describes a method for reclaiming chromium hydroxide from tannery wastes by agitation and continuous filtration through successive filtering zones.
In U.S. Pat. No. 4,086,319 to Jones there is disclosed a process which involves the incineration in an oxidizing atmosphere of tannery wastes at elevated temperatures between 800.degree. F. and 1200.degree. F., preferably between 900.degree. F. and 1000.degree. F. to convert the organic material to gases and the chromium material to ash. The process is directed only at the recovery of chromium from the ash by conventional leaching operations. A similar incineration method, for recovery of chromium, is disclosed by Smith "Chromium Recovery in the Tannery Alkaline Incineration", Leather Manufacturer, Vol. 95 (4), page 16 (1978).
The above-described method of Jones is primarily directed at the recovery of chromium. It is more or less efficient for the recovery of chromium values from tannery wastes. Thus, although U.S. Pat. No. 4,086,319 indicates that the products of incineration may be utilized to generate some of the heat requirements of the chromium recovery system, the disclosed process results in a relatively complete oxidation of the organic material which is required to render the gases suitable for discharge to the atmosphere.
Most importantly, however, the method does not enable an efficacious recovery of chromium in a form suitable for use in the tanning operation. The chromium utilized in the tanning process must be in the trivalent state. During the tanning process the chromium bonds to the protein (collagen) and exhibits trivalency in the bound form. Jones states that chromium may be recovered according to the patented process in the trivalent form (i.e., Cr.sub.2 O.sub.3) in the ash produced by the incineration or combustion step. The chromium oxide is described as "stable" and "substantially insoluble."
Jones further discloses that chromium in the hexavalent form can be obtained if the combustion is carried out in the presence of basic carbonate. It is further disclosed in the patent that lowering the combustion temperature or effecting incomplete combustion of the organic material still results in the formation of substantial amounts of insoluble, stable chromic oxides.
It is an object of the present invention to provide a method for treating liquid and solid tannery, tanning and finishing process wastes to recover soluble trivalent chromium and potential energy for return to the operation. The process will also reduce environmental control and waste handling requirements.